1. Field of the Art
The present invention relates to an electrochemical device, and more particularly to an electrochemical device capable of not only measuring an oxygen partial pressure of a lean-burned exhaust gas, but also detecting an amount of unburned components of a rich-burned exhaust gas.
2. Related Art Statement
There have been known various electrochemical devices, each of which comprises an electrochemical cell using a solid electrolyte body. Such electrochemical devices are used, for example, as oxygen sensors to detect the oxygen concentration of an exhaust gas (combustion exhaust gas) produced by internal combustion engines of automotive vehicles. Typical examples of such oxygen sensors include an oxygen sensor which employs a mass of oxygen-ion conductive solid electrolyte such as zirconia ceramics, to determine he oxygen concentration according to the principle of an oxygen concentration cell. In the field of manufacturing such electrochemical devices, there has been an increasing trend of using an electrochemical cell of a laminar structure which comprises a planar solid electrolyte body and planar electrodes disposed in contact with a surface or surfaces of the planar body of solid electrolyte.
The above-indicated types of electrochemical sensors were originally developed and designed as oxygen sensors for sensing a measurement gas such as an exhaust gas which is produced as a result of combustion of an air-fuel mixture at the stoichiometric air-fuel ratio. Recently, however, it is proposed to use such oxygen sensors as so-called "lean A/F" sensors for sensing a lean-burned exhaust gas which is emitted in combustion of a fuel-lean air-fuel mixture, i.e., as a result of combustion with an excessive amount of air. Namely, the "lean A/F" sensor is capable of detecting the oxygen partial pressure of an oxygen-rich exhaust gas whose oxygen partial pressure is higher than that of the stoichiometric air-fuel ratio.
As a "lean A/F" sensor of such type, an electrochemical device is known, which has an electrochemical sensing cell which comprises: a first planar oxygen-ion conductive solid electrolyte body; a first and a second electrode of porous structure disposed on the first planar solid electrolyte body such that the first and second electrodes are spaced from each other; diffusion resistance means which has a predetermined diffusion resistance to a measurement gas and covers the first electrode so that the first electrode is exposed to the measurement gas diffused through the diffusion resistance means; means for introducing a reference gas having a predetermined oxygen partial pressure, for exposure of the second electrode to the reference gas; and means for applying an electric current between the first and second electrodes, to induce an electrode reaction for controlling an atmosphere adjacent to the first electrode. In this arrangement of the electrochemical device, the oxygen component of the measurement gas which has been diffused toward the first electrode through the diffusion resistance means with its diffusion resistance, is further moved toward the second electrode by means of a pumping action which takes place due to a flow of the electric current between the first and second electrodes. Since a voltage between the two electrode is changed abruptly when the amount of diffusion of the oxygen component corresponding to the oxygen concentration of the measurement gas is changed beyond a given limit, the value of electric current upon abrupt change of the voltage, that is, a limiting current is measured to determine the oxygen concentration (oxygen partial pressure) of the measurement gas.